Field of the Invention
The present invention relates generally to cardiac electrode assemblies, and more particularly to a bipolar myocardial electrode assembly adapted to be electrically and mechanically connected to heart tissue.
Electrical simulation of body tissue and organs as a method of treating various pathological conditions and providing appropriate therapies is quite common-place. Such electrical simulation necessarily requires some manner of making electrical contact with the body tissue or organ. One example of such therapy is the use of a pacemaker to provide electrical pulses through electrical wires, called leads, to cause the diseased heart of a patient to beat normally. Pacemaker leads have electrodes located on the distal end of the lead for contacting the tissue of the heart. In the case of a patient who is having surgery only to install a pacemaker and one or more leads, transvenous leads which enter the heart through a vein are typically utilized. Transvenous leads contact the heart tissue inside the heart, and do not require opening the chest of the patient in a procedure known as a thoracotomy for the leads to be installed.
However, in the case of a patient who is having open heart surgery and who will require cardiac pacing following the surgery, electrical leads may be placed on the outside of the heart on the epicardial surface or into the myocardial tissues during a thoracotomy, during which procedure the heart tissue is exposed. Such electrodes are typically referred to as myocardial or epicardial leads. Examples of procedures in which the patient's heart will be exposed by a thoracotomy include a cardiac artery bypass procedure, corrective cardiac surgery for congenital defects, heart valve replacement, and the installation of an automatic implantable cardiac defibrillator (AICD). If following the procedure the patient will require cardiac pacing, it is expedient to install pacing electrodes while the patient's heart is exposed.
There are additional situations in which the installation of transvenous pacing leads will not be possible. These situations include the case when the vein which would be used is damaged or too small, or the situation in which a physical or anatomical anomaly prevents the placement of a transvenous lead within the heart, or in the case where an artificial heart valve is present. In these cases, the use of a myocardial lead is necessary.
A number of different myocardial electrode structures have been developed, as have various techniques for implanting those electrode structures into the myocardial tissue of the heart. Typically, myocardial electrodes are attached to the exterior of the heart, so that they may be stimulated by a cardiac pacemaker which is also implanted within the patient's body. The electrodes of myocardial leads are installed by being brought into electrical contact with the heart, and then sutured and/or secured in place with some fixation means.
It is generally preferable to use a plurality of myocardial electrodes rather than a single electrode (and the pacemaker can as the other electrode) to prevent inadvertent stimulation of skeletal muscle or other tissue from occurring during cardiac pacing. Thus, at least two electrodes are needed to provide effective stimulation and sensing of the heart. Use of two electrodes both mounted on the heart is known as bipolar pacing. It is undesirable to use two distinct myocardial leads each containing a single electrode in that the number of leads as well as the surgical steps required to implant a plurality of electrodes are increased. As the number of leads is increased, the size of the surgical opening into the patient's body, the number of wounds into the heart and the resultant trauma of the entire surgical procedure are increased. Thus, rather than installing two separate unipolar myocardial leads, it is preferable to install a single bipolar or multipolar myocardial lead.
One such bipolar myocardial lead is disclosed in U.S. Pat. No. 4,010,758, to Rockland et al. The Rockland et al. lead utilizes a bipolar body tissue electrode having a first electrode in a helical (screw-in) configuration, and an annularly configured second electrode surrounding the helical first electrode. A sheet of netting is used in the Rockland et al. device to enhance fibrotic growth.
A significant problem associated with cardiac electrode assemblies is that prior myocardial lead designs have not used electrodes designed to provide edge effects (increased electrical field density at an electrode caused by a conductive surface meeting a conductive edge of the electrode) efficiently. An increase in edge effects can increase the intensity and focus of electrical fields and provide for appropriate cardiac muscle stimulation with significantly less energy consumption. It is an objective of the present invention to use edge effects to greater advantage than is done in the art.
A related problem associated with cardiac pacemakers relates to the fact that pacemakers use an internal power source (typically a battery) with a limited life. After an extended period of time, such as five years, the implanted pacemaker must be removed (because its power source is depleted) and replaced with a new pacemaker. Therefore, it is an objective to minimize the electrical current drain on the power source by appropriate design of the pacemaker's electrodes to provide for reduced stimulation voltage, while still maintaining the strength of the resultant electric field through the myocardium at a sufficiently high level to stimulate the heart.
A further problem associated with cardiac electrode assemblies is that prior art myocardial lead assemblies are relatively rigid and do not conform to the shape of the myocardial surface and its motion during cardiac contractions and rest periods. Thus, the area of contact between myocardial lead electrode(s) and its (their) edges, and the heart tissue was not either complete or intimately mobile. It is an objective of the present invention to provide a myocardial lead which ensures reliable and intimate, non-traumatic contact with the heart tissue, and which will maintain that contact during both heart contractions and rest periods.
A still further problem associated with previously known myocardial electrode assemblies is that they do not provide suitable means, other than a single screw-in helix (or a single barb), for securing the assembly to the heart tissue. Although a screw-in helix electrode is a fairly good means to attach an electrode assembly to heart tissue, under certain circumstances it is desirable to provide additional securing means to prevent disattachment of the lead's electrode assembly from the heart. Accordingly, it is an objective of the present invention to provide improved means for securing the electrode portion of the lead to the heart.
An additional problem associated with cardiac electrodes assemblies in the prior art is that they did not have additional pacing electrodes or integral sensor electrodes, but rather required additional separate electrodes to be connected to the heart tissue. The connection of separate additional electrodes to the heart tissue increases trauma to the heart and increases the risk of a failure with an increased number of connections to the heart. Accordingly, it is an objective of the present invention to optionally provide such additional electrodes for stimulation and sensing in a single myocardial lead.
Finally, it is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.